1. Field of the Invention
This invention relates to the field of radio frequency identification (RFID) and, more particularly, to improved reliability of communication between any RFID reader and tag in an RFID system.
2. Discussion of the Prior Art
RFID systems exhibit unique communications reliability issues. Communication in RFID systems is normally initiated by the transmission of a carrier signal from the reader to the tag. However, this communication could also be from the tag to the reader. Reliability issues arise due to the relatively high instability of the RF field strength, the relatively error-prone decoding of received signals at both the reader and tag, and the uncertainty in the timing of the carrier detection. RF field strength may become insufficient and/or may fluctuate frequently. The RF field strength instability is evidenced, for example, if the carrier signal amplitude or quality is degraded by, for instance, movement of the reader relative to the tag. Degraded carrier signal quality can cause the integrity of the transaction to be lost, resulting in an incorrectly received message and, if the error is detected, necessitating retransmission of the entire transaction or message. In addition, it is common for RFID systems to be operated close to the limits of their range, so that even in the absence of movement, the RF field strength may vary, causing a high bit error rate of the receiving device, either the reader or the tag.
Another communications reliability issue arises due to the uncertainty in the timing of carrier detection relative to the start of reading or transmission. This uncertainty can give rise to a shifting, or partial shifting, of a carrier or message acknowledgement bit stream, and can cause the failure of standard message parsers, which may not be equipped to handle this shifting bit stream. In passive tags, where the carrier also supplies energy and/or a signal path for the response of the tag, this shifting can be acute, resulting not only in misinterpretation of the reader's request, but also in an inappropriate response by the tag, causing a two-way communications failure. In active tags, the communication requests between the reader and the tag may be more complex, requiring specific tasks for the reader to validate the results received from the tag and for the tag to validate requests from the reader.
Communications reliability in RFID systems is addressed in WO 2004/109583 for “Wireless Communications Device, System and Communications Method”. A method to overcome RF interference and distinguish signals from multiple RFID tags within the range of one reader is disclosed, including a technique for frequency-shift-key (FSK) encoding for use in the communications protocol of the RFID reader/transponder. The application discloses that its system can be used with transceiver sensor data.
A communication system protocol used to distinguish among multiple RFID tags is disclosed in U.S. Pat. No. 6,452,504 B1 for “System and Method for Communication with Radio Frequency Identification Tags Using TOW Message DFM Protocol”. This patent describes a protocol for use in a home monitoring environment, using directional location based on reception of a three-phase AC signal. The protocol, which is somewhat unique to the home monitoring environment, requires the use of an antenna on the RFID transponder. The patent discloses actually estimating the relative location and/or direction of several transponders with respect to a given transmitter. The patent discloses sensors and RFID tags among its applications.
Present RFID systems address the communications reliability issue in a variety of ways, such as by increasing the reader's carrier signal strength or amplitude, by using large or more efficient antennas, by using data encoding schemes, such as differential bi-phase or Manchester encoding, within the protocol, or by continuously retransmitting. These techniques frequently result in high energy consumption at the reader, often limiting the use of portable readers, and also in RF interference of the RFID system when its signal is injected into receivers, e.g., intermediate stage receivers, for other RF systems in a location. All of these methods reduce throughput and may require manual intervention by the user in order to achieve reliable transmission. Presently deployed RFID applications are predominantly single-reader, single-tag applications, with “passive” RFID tags; the present invention is particularly suitable for multi-reader, multi-tag environments and/or for “active” tags. An “active” tag is one that responds with more than simply “identity” information, and may respond to different reader commands with different types of messages, and/or carry on a multi-step “transaction” with the reader.